The present invention relates to signal reproducing systems, particularly for high frequency signals stored on record discs.
Mechanical pickups are known for scanning disc-shaped record carriers, or picture records, on which television pictures, for example, are stored. In contradistinction to the mode of operation for phonograph record pickups in which the pickup element is deflected by the recorded signal oscillations, the pickup element remains at rest when scanning such picture records and the deformable picture record undulations exert a more or less strong pressure force on the pickup element. The basic picture record arrangement is described, for example, in U.S. Pat. No. 3,652,809, issued to Gerhard Dickopp et al.
In a known device in which the signal track is constituted by undulations in the walls of a groove, the pickup element is an asymmetrical skid whose leading edge, based on its direction of movement relative to the tracks, has a flat shape and whose trailing edge has a steeply ascending shape. Good pickup results are obtained if:
1. THE STEEPLY ASCENDING PORTION OF THE SKID FORMS AN ANGLE OF ABOUT 90.degree. WITH THE SURFACE OF THE RECORD AND HAS AS SMALL AS POSSIBLE A RADIUS OF CURVATURE SO THAT THE UNDULATION PEAKS OF THE PICTURE RECORD CAN UNIMPEDEDLY SNAP UPWARD AGAIN WHEN THEY LEAVE THE REAR END OF THE SKID; AND
2. THE CONTACT SURFACE OF THE SKID IS NOT TOO LARGE IN THE DIRECTION OF THE GROOVE SO THAT THE UNDULATION PEAKS RELEASED BEHIND THE REAR OF THE SKID CAN HAVE AN INCREASED INFLUENCE WITH RESPECT TO THE PROPORTIONAL CHANGE IN PRESSURE ON THE SKID UPON DEPARTURE OF AN UNDULATION PEAK FROM CONTACT THEREWITH.
A certain drawback of mechanical pressure scanning is the wear inevitably experienced by the skid, due to which its contact surface becomes wider and its sharp rear edge is worn down. The widening of the skid will have an annoying influence only when the skid no longer rests on the groove wall undulations carrying the recorded information but rather rests on the surface portions outside the groove. The wear on the steep rear edge creates problems much earlier, however.
Methods for scanning record carriers employing light radiation or similar radiation are also known. In this case, the signals are stored in the form of different degrees of darkening of the carrier surface. The quantity of light which is either reflected or transmitted, depending on the degree of darkening, excites a light receiver whose electrical output value constitutes a representation of the signal. There exists the difficulty in the case of this method that copies of the carrier can be obtained only by way of photochemical processes or similar procedures, but not by the simple and inexpensive pressing or stamping process known for fabricating phonograph records.
It is also known to optically scan carriers which contain the signal recording in the form of deformations or undulations on their surface. Difficulty has been encountered in this case in bringing the portion of the light reflected by the surface into an unambiguous relationship to the size and type of deformations involved. This requires complicated optical systems which result in poor utilization of the reflected light. Such systems thus have a low energy utilization efficiency and a low signal to noise ratio.
A system is also known which evaluates the deformations in a picture record, which correspond to the signal values, by means of light radiation, or similar radiation, and a radiation receiver. For this purpose a slit aperture and the surface bearing the deformations are arranged in the path of the beam at such a spacing from one another that in the plane of the slit there results, in dependence on the curvature of the surface bearing the deformations in the direction of the relative speed of the carrier and with respect to the collimated scanning beam, a change in density of the radiation emanating from this surface, which change at least qualitatively represents the path of the curvature. A system of this type is disclosed in German Offenlegungsschrift (Laid Open Application) No. 1,903,822 and in U.S. application Ser. No. 295,011, filed by Gerhard Dickopp on Sept. 29, 1972.
It is known in this connection, in order to maintain the pickup system on a perfect track with respect to the associated signal track and to guide it vertically when the carrier undergoes vertical movements, to provide a guide element which engages, together with a slide element, in a groove of the carrier containing the deformations and includes a member which is optically effective for the particular type of radiation employed and which takes a substantial part in the evaluation of the changes in light density effected by the deformations. This member may be, for example, a slit aperture or a lens as disclosed in U.S. Pat. No. 3,654,401, issued to Gerhard Dickopp.
The previously suggested optical pickup systems, however, have the drawback that when they are used for carriers having a very high storage density the beam cross section must be made so small that it attains the order of magnitude of the wavelength of the radiation. This makes it very difficult to obtain good focussing of the beam due to diffraction phenomena. Thus limits are set for the information storage density.